Nuclear imaging systems typically employ tissue or condition selective radioisotopes which are injected into the patient, and whose progress through the patient, or specific organs or systems in the patient, is monitored by a scintillation or gamma camera (also popularly known as an "Anger camera" after the pioneer thereof). The camera, typically suspended over the region in interest, possesses a scintillation crystal which converts collimated radiation from the isotope within the patient into a flash of light. A cluster of photomultiplier tubes (PMTs) is arrayed above the scintillation crystal, typically across one or more layes of glass or light transmissive plastic. Each tube converts the light received from the scintillation flash into an electrical signal. In turn, these electrical signals are processed in known fashion to locate the origin of the scintillation event (and hence of the nuclear event in the patient); and the aggregate of many such events is accumulated to produce an image.
Conventionally, the scintillation event is identified and located rather precisely by detecting the light intensity from the event at a plurality of next adjacent PMTs overlying the event. That is, since the PMTs accept light generally in proportion to the solid angle subtended by the PMT face relative to location of the scintillation event, such scintillation event in the crystal will produce a set of signals at overlying PMTs which is characteristic of the location of the event. Conventional analog and digital image processors are equipped to locate the event or "centroid", as the peak of a two-dimensional bell curve of intensity signals from the array of PMTs above the crystal. Also in conventional fashion, then, the actual image is displayed from the cumulative identification and location of the centroids.
As is known in the art, scintillation cameras experience inaccuracy (generally subdivided into energy, linearity, and flood constituents) for a number of reasons relatively inherent to the design. First, the discrete nature of the PMTs, as compared with the continuous nature of the crystal, ends an inherent degree of inaccuracy. Secondly, the tubes themselves are relatively large compared to the area which in the aggregate they monitor, contributing a degree of undersampling. Thirdly, even modern tube manufacturing and quality assurance procedures allow some degree of nonuniformity among individual PMT operating characteristics, thereby yielding a further source of inaccuracy. Finally, the variability in thickness of the glass layers separating the scintillation crystal from the PMTs produces a like gradient in tube response. Together, these factors lend a degree of inaccuracy in the process of locating the position of a scintillation event. Moreover, design approaches to compensate for one effect or another often entail tradeoffs regarding other functions and features of the system, and/or regarding overall system economics.
It is a primary object of the present invention to provide methods for correcting linearity or spatial distortion in scintillation cameras, that is, to provide methods for precisely locating the actual position of a nuclear event based on its apparent position as evaluated by the camera and system.
The prior art has allocated significant effort to correction of spatial or linearity distortion (along with the companion energy and flood corrections). In particular, there has existed in the prior art an appreciation that spatial distortion occurs, and that it is possible to calibrate individual cameras to compensate for the particular spatial distortion involved. In U.S. Pat. No. 3,745,345 to Muehllehner, the camera head is covered by a lead mask having a grid of pinhole apertures therethrough. A sheet source of radiation causes each aperture uniformly to illuminate the scintillation crystal therebelow, and the camera records the apparent location of the event in the crystal. The difference, of course, between the actual pinholes and the apparent events as located by the camera, is representative of spatial distortion at the respective locations on the camera face, and based upon this Muehllehner defines a correction factor for each such point in a stored array. Then, "on the fly", Muehllehner assembles an uncorrected digital image or map, the counts from which are then redistributed corresponding to the spatial correction factors in storage. There results a spatially redistributed map based on the stored correction factors. Muehllehner also teaches the use of the stored correction factors for correction of the points therebetween by interpolation, and the correction of each scintillation event individually in accordance with the stored factors.
U.S. Pat. No. 4,212,061 to Knoll et al. teaches another, somewhat more sophisticated method of spatial correction. For calibration, Knoll uses a lead mask having parallel slit apertures, through which the camera is exposed to a sheet radiation source, first with the mask in one orientation and then with the mask rotated by 90.degree.. For each such exposure orientation, there is developed a series of transverse peak measurements at select intervals. An analytical polynomial expression is generated to represent event coordinates between calibration intervals, preferably by use of a cubic spline polynomial expansion. Each orientation exposure thus produces one of a pair of calibration coordinates, which in turn permit direct correspondence to associated time spatial coordinates. On the fly, each individual event is provided with correction based on the apparent location of the event, and the actual event location is determined by linear interpolation of the true coordinates associated with the stored correction factors.
Later, units commercially distributed by the assignee hereof under the trade designation "Omega" have utilized similar techniques to beneficial effect. In the Omega brand cameras, pinhole aperture masks are used in the calibration steps to develop a correction vector grid, which is stored in a look up table. On the fly, apparent events are corrected to the actual location by adding a correction vector which is determined by bilinear interpolation of the stored values.
The above described methods of the prior art have significant beneficial effect on spatial distortion in scintillation cameras. Nevertheless, systems which employ these methods still display substantial artifacts which in appearance can resemble a checkerboard style tonal gradient which overlays the imge in general correspondence with the location of the spatial linearity correction vectors.
It is an object of the principles of the present invention to provide spatial linearity correction while yet avoiding the artifacts heretofore inherent in the schemes of the prior art.